Nuclear magnetic resonance (NMR) has become an increasingly powerful and convenient tool for both medical diagnostics and medical research. The use of NMR solves the problems attending many applications where it is desirable that large areas of a patient's body be scanned simultaneously without the movement of either the patient or of large pieces of equipment. In order to accomplish this, relatively large uniform fields (above 1 kOe) are required over volumes sufficient to accommodate most human torsi. However, it is desirable in many medical diagnostic and experimental procedures to perform tests or studies without the application of magnetic fields produced by electrical solenoids, bulky power supplies and the like.
In a comparison of the three basic types of magnet systems (resistive, permanent magnet, and superconductive), it was found that the permanent magnet type yields the best magnetic field/cost ratio in the low field (1.5-3.0 kOe) realm. Superconductive magnet type NMR diagnostic devices have been constructed, and while found suitable for the intended use, they have been disadvantaged by the heavy weights of the magnets, and the necessary addition of liquid cryogens for cooling purposes.
The above-mentioned problems were also addressed in the co-pending patent application Ser. No. 420,745, to Leupold et. al., filed Oct. 12, 1989 which is incorporated herein by reference. In this co-pending application there is disclosed a magnetic device for use in nuclear magnetic resonance diagnostics comprising a longitudinally extending magnet element having transversely extending flux lines and forming a diagnostic working space conforming generally to the shape and size of a preselected diagnostic target. Cladding magnets, composed of rigid permanent magnet material, are disposed exteriorly along the longitudinally extending magnet element to confine the flux within the work space. The cladding magnets taper in thickness from the top and bottom of the sides of the magnet element cancelling out the flux lines that leak or deviate to the outside of the structure. The magnetic orientation of the cladding magnets is perpendicular to the magnetic field within the work space. Bucking magnets and pole pieces are arranged at the ends of the magnet element to prevent end losses. The pole pieces, also known as the "yoke", are often formed of iron and are also advantageous in smoothing out the effects of nonuniformities caused by defects in the magnets. A more uniformly controlled flux of sufficient accuracy is provided from this clad-yoked magnetic structure.
Unfortunately, since iron is a passive magnetic material (as opposed to rigid magnetic material), its magnetic orientation may change in the presence of other electric and magnetic fields. As a result, when this structure is used in, for example, NMR imagers, the radio wave source thereof must be placed within the diagnostic working space of the magnetic structure. It is then necessary to construct bulkier, more massive structures with bigger cavities. Furthermore, the highly conductive pole pieces (or yoke) conduct eddy currents that can interfere with and distort NMR signals. Therefore, it is desirable to provide permanent magnet structures, particularly for NMR imaging, without iron pole pieces or yoke, so that more compact structures providing very large magnetic fields may be constructed.
Reference is further made to two articles authored by Abele et al., inventors herein. The first article "Applications Of Yokeless Flux Confinement" was published in the J. Appl. Phys. 64(10), Nov. 15, 1988. The article sets forth a permanent magnetic flux source of square cross-section fabricated of rigid magnetic material. The flux source comprises triangular sections with predetermined magnetic orientations arranged to form the square cross section. Magnetic fields of the order of the remanence of the material can be produced, which is a significant improvement over the previously discussed clad-yoked magnetic structure.
The second article, also by the inventors herein, is entitled "Compensation Of Non-Uniform Magnetic Properties Of A Yokeless Permanent Magnet" (IEEE Transactions On Magnetics, Vol. 25, No. 5, Sept., 1989). This is an improvement on the flux source of the first paper and on which this invention is based. A means for compensating the nonuniform properties of the magnet structure comprises magnetic dipoles positioned symmetrically within the cavity of the magnetic structure. As a result, a highly uniform magnetic field is achieved within the cavity of the structure.